1. Field of the Invention
The present invention relates to an optical disc apparatus for recording and reproducing an optical disc by suppressing laser noise of a laser diode.
2. Description of the Related Art
Japanese Patent Laid-open Publication (JP-A) No. 2003-257072 gives description of the optical disc as follows. In recent years, the digital versatile disc (hereinafter referred to as DVD) attracts much attention as a high-density optical disc capable of recording a large amount of digital information. However, as the capacity of information grows larger, realization of a higher density optical disc is sought. Note here that in order to attain higher-density recording than that of the DVD, it is necessary to record a smaller mark on a recording layer than that of the DVD. For this purpose, it is necessary to shorten a wavelength of a light source and enlarge the numeral aperture (hereinafter referred to as NA) of an objective lens. The DVD uses a laser of 660-nm wavelength as a light source and a lens of 6.0 NA as an objective lens. The use of a blue laser, for example, of 405-nm wavelengthas a light source and the objective lens of 0.85 NA makes it possible to attain a recording capacity about 5 times larger than the DVD. In addition to this, along with higher output of a blue laser in recent years, for the purpose of attaining higher recording capacity than the so-called one-layer disc with a single recording layer, a multi layer disc with two or more recording layers is being developed. For example, if a disc with two recording layers is realized, the storage capacity will be about 10 times the DVD. However, in the conventional high-density optical disc apparatus, since various stress margins at the time of reproduction are severer than the DVD, quantum noise of a blue laser acting as a light source poses a problem. This quantum noise can be controlled to be low if the output laser power is increased, which is a characteristic of a laser diode acting as a light source. However, if the output laser power is increased, a power of the laser that is converged and applied onto the recording layer (hereinafter referred to as irradiation power) will increase, causing degradation of the recording layer and erasing of data.
JP-A No. 2004-220744 discloses that a problem of “providing an optical head and an optical recording medium drive apparatus that can attain excellent recording and/or reproducing characteristic by successfully suppressing increase of laser noise in a light source even at a high temperature above the normal temperature” is solved by using a configuration such that “in the case where a laser diode is used as a light source, when a temperature becomes high, increase in the laser noise is suppressed and accordingly the reproduction characteristic is not influenced because higher the temperature, smaller the optical coupling efficiency is made, and consequently emission output of the light source is made larger.